Method of making Carbon/Ceramic matrix composites

ABSTRACT

A method of making high performance friction materials with tailored levels of a ceramic hard phase to achieve optimum thermal conductivity, friction coefficient and wear performance of composite brake materials. In accordance with one method of the invention specific end-use application friction requirements are satisfied by tailoring the level of carbon in a selected carbon/carbon preform, heat treating the carbon/carbon composite preform, thereby affecting thermal conductivity so as to optimize overall braking performance prior to ceramic processing and by selecting an optimum level of ceramic hard phase to achieve satisfactory friction disc wear life and friction characteristics of the braking material.

This is a Continuation Application of co-pending U.S. application Ser.No. 11/034,020 filed Jan. 11, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to carbon/ceramic matrix composites andthe method of making same. More particularly, the invention concernscarbon/ceramic matrix composites for use in friction applications.

2. Description of the Prior Art

With the rapid advance of aircraft, nuclear and aerospace and hightemperature technologies there is an ever-increasing need for new typesof high strength composite materials that are capable of reliablywithstanding high temperatures and pressures. Additionally, new methodsare required to expeditiously fabricate these materials into articlessuch as aircraft brake discs.

Carbon/carbon composites are the state-of-the-art friction materials foraircraft brake applications. The majority of today's commercial jets andmilitary fighters are equipped with carbon/carbon brakes.

Carbon fibers, by design, have well oriented crystalline structurealigned along the axis of the fiber. They exhibit good strength andstiffness in the fiber direction.

The ability to combine different carbon fibers with different types ofcarbon matrices to form a single light weight, economical and functionalcomposite is one of the key reasons for the successful application ofcarbon/carbon for aircraft brake use. The type and distribution ofcarbon fiber, the crystalline structures of various carbon matrices, theratio between soft and hard carbon in the matrix, and the overallcomposite thermal conductivity all have an impact on final brakeperformance.

Carbon/carbon was first proposed as an aircraft frictional material inthe beginning of 1970s. By the end of 1970s, carbon/carbon brakes werethe standard equipment for advanced fighters such as F-14, F-15 and thesupersonic Concorde.

Generally speaking, carbon/carbon brakes offer low wear and provideexcellent frictional performance at high energy conditions.Additionally, the use of carbon/carbon aircraft brakes significantlyadds safety and increases payload.

Prior art carbon/carbon aircraft brakes are generally composed ofmultiple full-circle rotors and stators of the same material.

The unique friction properties of structural carbon/carbon compositebrake materials have now been fully established for multi-discrotor/stator braking systems for commercial and fighter aircraft, aswell as for caliper/single disc applications for helicopters,industrial, automotive and train braking applications. Carbon/carbonbraking materials are currently manufactured in large volume productionquantities, especially for commercial and military aircraft. Wear lifeand friction coefficients are at predictable levels and cannot besignificantly influenced by carbon/carbon processing conditions.

The technology for densifying carbon fiber substrates by liquid pitch orresin impregnation, carbonization and graphitization or chemical vaporinfiltration of pyrolytic carbon, with subsequent composite heattreatment is fully established and a variety of carbon/carbon productsare routinely manufactured including complex aerospace components, hightemperature furnace hardware, components for the Semi-ConductorIndustry, brake discs for commercial and military aircraft, as well asfor automotive and other commercial applications.

Densification by the chemical vapor infiltration (CVI) process is themost popular manufacturing process in the industry to date for themanufacture of carbon/carbon composite friction materials for aircraftbraking systems. Fiber volume for the carbon substrates may range from20%-30%. Depending on initial carbon fiber density, fiber volume andnumber and length of pyrolytic carbon infiltration furnace runs, thefully densified carbon/carbon composite product may range in densityfrom 1.5 g/cc to 1.85 g/cc.

Two of the early patents concerned with carbon/carbon aircraft brakesand the methods for making the brake discs, namely U.S. Pat. Nos.3,895,084 and 3,991,248 were issued to the present inventor. Thesepatents describe unique substrate optimization techniques as well asnovel methods for accurate control of product shape, cross-sectionalconfiguration, density, fiber volume and internal fiber orientation.

One of the drawbacks of prior art carbon/carbon brakes is that theytypically yield lower frictional coefficients, which tend to vary widelyat different speed and landing energy. Carbon/carbon is also susceptibleto oxidation damage, which not only degrades its structural integrityover long-term usage, but also promotes accelerated wear.

In the past, considerable development work has been carried out todevelop a ceramic matrix composite (CMC) friction material that exhibitsimproved friction properties over carbon/carbon brake materials.

These efforts have largely focused on material systems that are based oneither silicon melt infiltration and carbide conversion, or pre-ceramicpolymer impregnation of carbon fiber mats, resulting in ceramic matrixcomposites after pyrolyzation. This work has to date, not beenparticularly successful for aircraft braking applications.

Another early development effort is described in U.S. Pat. No. 5,153,295issued to Whitmarsh et al., entitled “Carbosilane Polymer Precursors ToSilicon Carbide Ceramics”. This patent describes a process for thepreparation of compositions of matter which have potential utility asprecursors to silicon carbide (SiC) wherein the compositions areobtained by a Grignard coupling process starting fromchlorocarbosilanes. The precursors constitute a type of polycarbosilanethat is characterized by a branched, [Si—C].sub.n“backbone” comprised ofSiR.sub.3 CH.sub.2—, —SiR.sub.2 CH.sub.2—, .dbd.SiRCH.sub.2—, and.tbd.SiCH.sub.2—units (where R is usually H but can also be otherorganic or inorganic groups, e.g., lower alkyl or alkenyl, as may beneeded to promote crosslinking or to modify the physical properties ofthe polymer or the composition of the final ceramic product). A keyfeature of these polymers is that substantially all of the linkagesbetween the Si—C units are “head-to-tail”, i.e., they are Si to C.

Recently, considerable effort has been directed toward developingceramic matrix composites (CMC) that are specially aimed at aircraftbraking applications. Much of this work has been based on pre-ceramicpolymer impregnation of carbon fiber preforms that may undergo as manyas twelve polymer impregnations before the desired final density isreached. Typically the pre-ceramic polymer forms the ceramic matrixafter pyrolyzation between about 850° C. and about 1600° C.

One of the goals of the present invention is to further optimizecarbon/ceramic friction material by identifying key material processvariables and systematically correlating the resulting carbon/ceramicmatrix composites with brake performance and to develop reproducible andcost effective processing steps to fabricate carbon/ceramic matrixcomposites for future brake applications.

DISCUSSION OF THE INVENTION Definition of Terms

a. Carbon Fibers: Carbon fibers are fibers produced by the heat treatingof both natural and synthetic fibers of materials such as, for example,wool, rayon, polyacrillonitrile (PAN) and pitch at temperatures on theorder of 1000° C. or more.b. Graphite Fibers: Graphite fibers are fibers produced by theheat-treating of carbon fibers at graphitizing temperatures on the orderof 2000° C. or more.c. Pyrolytic Carbon: Pyrolytic carbon, as the term is used herein,refers to the carbon material which is deposited on a substrate by thethermal pyrolysis of a carbon-bearing vapor over the temperature rangeof 800° C. to 1200° C.d. Pyrolytic Graphite: Pyrolytic graphite is a trade name which has beengiven to carbon deposited from a hydrocarbon gas over the temperaturerange of 1750° C. to 2250° C. It is a specific high temperature form ofpyrolytic carbon.e. Pyrolytic Carbon Infiltration: Pyrolytic carbon infiltration is aterm used to describe the carbon densification processing of porousfibrous and particulate substrates.

One aspect of one form of the process of the present invention involvesthe chemical vapor infiltration of pyrolytic carbon into a carbonfibrous preform and the tailored impregnation of a pre-ceramic polymeror CVI ceramic product at various stages of the carbon/carbondensification process to achieve the required friction coefficient andwear performance. Additionally, particulate fillers may be used in thisprocess to further affect the rate of densification, thermalconductivity and friction properties.

When using the pre-ceramic polymer process as source for the ceramicfriction modifier, depending upon polymer mass for a given furnace run,heat-up rates and temperature holds for producing an acceptable ceramicmatrix composite may vary. Typically, the polymer is cured or crosslinked at a temperature of about 400° C. in an inert atmosphere.

Pyrolysis is accomplished at about 100° C. above the maximum expectedend-use temperature or 850° C., whichever is greater. In carrying outthe method of the invention, an optimum pyrolyzation rate has to becarefully followed based on the mass of the parts and the level ofpre-ceramic polymer used. Additionally, the ramp rate for pyrolyzationhas to be carefully selected to avoid blistering or delamination ofparts.

Pyrolysis done at 1000° C. will yield about an 80-85% ceramic mass.However, when pyrolysis is carried out at about 1600° C. the mass yieldwill change to about 75-80%. Pyrolysis of AHPCS and relatedhydridopolycarbosilanes to temperatures of about 850° C. to about 1,000°C. will result in a ceramic that is amorphous (non-crystalline) siliconcarbide (SiC), whereas pyrolysis carried out to 1600° C. will result ina ceramic that is a crystalline beta silicon carbide (SiC).

In carrying out the carbon/ceramic friction modifier process of thepresent invention, any of the currently practiced carbon fiber substrateconstructions for the manufacture of carbon/carbon composites can beemployed.

In order to achieve optimum levels of a ceramic hard phase versus softcarbon/carbon phase the carbon fiber substrate is infiltrated withpyrolytic carbon followed by high temperature heat-treatment to reachthe required initial carbon composite density, that is, porosity level,suitable for subsequent ceramic processing and to allow the requiredratio of ceramic hard phase versus soft carbon/carbon phase to beachieved.

Depending on the density and open porosity of the carbon/carboncomposite and the desired ultimate level of a hard phase, the carboncomposite is then infiltrated with a hard carbide or nitride from avapor phase or liquid impregnated with a pre-ceramic polymer. For thepre-ceramic polymer process, the amount of the hard phase will betailored for the desired end-use of the material by the number ofpolymer impregnation and pyrolyzation runs used. For example, inaccordance with the method of the present invention, in a first, or TypeA process sequence, two to three impregnation runs are accomplished. Thefirst friction discs thus produced are dynamically tested to determinetheir strength and friction characteristics.

This done, a second, or Type B process sequence will be accomplishedwherein five to seven impregnation runs are accomplished. The secondfriction discs produced by this process will then be tested to determinetheir strength and friction characteristics and these characteristicswill be compared with those of the first friction discs. Finally, athird, or Type C process sequence will be accomplished wherein seven tonine impregnation runs are accomplished. The third friction discs thusproduced will be carefully tested and their strength and frictioncharacteristics will be compared with those of the first and seconddiscs.

The carbon fiber substrate will be infiltrated with pyrolytic carbon toreach the required initial carbon composite density, i.e., porositylevel, suitable for subsequent ceramic processing and to allow therequired ratio of ceramic hard phase versus soft carbon/carbon phase tobe achieved.

Once the desired carbon composite density is achieved, the carbon/carbondisc preforms may be heat-treated to between about 1600° C. and about2500° C. to impart the required end-use thermal properties. A final CVIcarbon infiltration run after all ceramic processing has been completed,will bring the composite to its optimum strength while, by the nature ofthe infiltration and coating process, the pyrolytic carbon willreinforce the ceramic phase, locking the hard ceramic product in placewithin the open porosity of the carbon/carbon composite and aid inachieving the desired thermal conductivity level in the final product.

It is to be understood that the selected fiber preform and itsconstruction determine the structural (tensile) strength of thecarbon/ceramic matrix composite and the distribution of both fiber andmatrix. Molded chopped fiber preforms, two-dimensional lay-ups andthree-dimensional, needled carbon fiber preforms are proven and havebeen widely used for aircraft brake application.

Due to the nature of the pore structure in the various preform types,each preform type responds differently to different densificationprocesses.

It is also to be understood that Silicon Carbide (SiC) matrix compositesgenerated from different densification processes exhibit differentpurity and crystalline structure which translates to different thermaland physical properties. For example, 1000° C. pyrolyzed, SiC derivedfrom the pre-ceramic polymer process (Starfire System) is amorphous innature. It can be converted to beta phase after 1600° C. postprocessing.

Pre-ceramic polymer impregnation processing and pyrolyzation, generallyspeaking, provides more uniform densification through the thickness ofthe composite. Chemical vapor infiltration (CVI) deposited SiC on theother hand is very crystalline. SiC derived from melt infiltrationdepends heavily on the process and filler composition. Pure unreactedsilicon metal is often present in the final matrix. Being a lowermelting material, free silicon is considered as undesirable for brakeapplications especially under high energy input condition for aircraft.

A fibrous substance for carbon/carbon densification may employ rayonprecursor fibers, pitch or PAN (polyacrillonitrile) fibers. Substratesmay be constructed using chopped pitch or PAN fiber molding compoundswith phenolic resin or laminated graphite cloth prepreg, again withphenolic resin.

The substrate can also be assembled from dry graphite cloth layers andcompacted in a fixture to achieve a certain fiber volume. Similarlyfiber mat layers, needled or stacked, can be compressed duringcarbonization and subsequent densification to achieve the required fibervolume for a given application.

Carbon/carbon densification can be based on a multiple pitch or phenolicresin low pressure impregnation and carbonization (Lopic) process. Hightemperature graphitization heat-treat runs are inserted aftercarbonization.

Heat treating the composite will cause fiber and matrix shrinkage,opening up more porosity to more readily facilitate subsequentadditional liquid impregnation and carbonization runs to ultimatelyreach composite density levels of 1.6 g/cc to 2.1 g/cc.

Any of the currently practiced carbon fiber substrate constructions forthe manufacture of carbon/carbon composites can be employed for thecarbon/ceramic friction modifier process of the present invention.

This process is based upon chemical vapor infiltration of pyrolyticcarbon into a carbon fibrous preform in addition to tailoredimpregnation of a pre-ceramic polymer or CVI ceramic product at variousstages of the carbon/carbon densification process.

Additionally, particulate fillers may be used in the pre-ceramic polymerimpregnation process to further affect friction properties and rate ofdensification.

Once the desired carbon composite density is achieved, the carbon/carbondisc preforms are heat treated to between 1600° C. to 2500° C. to impartthe required end-use thermal properties. A final CVI carbon infiltrationrun after all ceramic processing has been completed, will bring thecomposite to its optimum strength level while, by the nature of theinfiltration and coating process, the pyrolytic carbon will reinforcethe ceramic phase locking the hard ceramic product into place within theopen porosity of the carbon/carbon composite and aid in achieving thedesired thermal conductivity level in the final product.

The deposition temperature required, CH₄ gas flow rate and pressurelevel for CVI carbon densification are well established. The level offinal pyrolytic carbon placed into the pores of the carbon/ceramiccomposite is controlled by the length of the CVI carbon run and is basedon the desired ratio of ceramic hard phase versus soft carbon/carbonphase.

SUMMARY OF THE INVENTION

It is an object of the present invention to optimize carbon/ceramicfriction material candidates by identifying key material processvariables and then systematically correlating the resultingcarbon/ceramic matrix composites with brake performance.

More particularly, it is an object of the present invention to create anew family of high performance friction materials with tailored levelsof a ceramic hard phase to allow optimization of thermal conductivity,friction coefficient and wear performance of carbon/ceramic compositebrake materials.

Another object of the invention is to develop reproducible and costeffective processing steps to fabricate carbon/ceramic matrix compositesfor future brake applications.

By way of brief summary the methods of the present invention are adaptedto satisfy specific end-use application friction requirements bytailoring the level of carbon in the carbon/carbon preform, heattreating the carbon/carbon composite preform, thereby affecting thermalconductivity to optimize overall braking performance prior to ceramicprocessing and by selecting a certain level of ceramic hard phase toachieve the friction disc wear life and friction coefficient desired.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron microscope image at a magnification of 100of a 3D carbon/carbon preform after densification with pre-ceramicpolymer and conversion to Beta-Silicon Carbide at 1600° C.

FIG. 2 is a scanning electron microscope image at a magnification of2000 of a typical partially CVI carbon densified 3D needled PAN fibercarbon/carbon preform, following pre-ceramic polymer impregnation andcarbide conversion at 1600° C.

FIG. 3 is a scanning electron microscope image at a magnification of 100of a typical partially CVI carbon densified 3D needled PAN fibercarbon/carbon preform, following additional densification by CVI Siliconcarbide process.

FIG. 4 is a scanning electron microscope image at a magnification of2000 of a typical partially CVI carbon densified 3D needled PAN fibercarbon/carbon preform, following CVI SIC processing.

FIG. 5 is a scanning electron microscope image at a magnification of5000 of a CVI carbon densified, 3D needled, PAN fiber mat, followed byCVI SIC processing.

DESCRIPTION OF THE INVENTION Carbon Fiber Preform Fabrication

In carrying out the methods of the present invention, various types ofcarbon fiber substrates are used.

One such substrate, namely a needled PAN fiber carbon mat substratecomprises a continuous filament, polyacryllonitrile (PAN) fiber towhaving a filament count of approximately 320K. The tow stabilizationtemperature is on the order of 200° C.-300° C. The substrate preferablyhas a fiber mat lay-up orientation of between about 0° and about plus orminus 60°. The fiber volume as needled, prior to carbonization, is onthe order of 50%.

Carbonization of the PAN fiber mat is accomplished in about a five-daycarbonization run and fiber volume after carbonization is approximately25%. After carbonization and prior to carbon/carbon densification, thefiber mat is vacuum heat-treated at a temperature of about 1700° C. Thecarbon fiber preform density after heat treatment is approximately 0.5g/cc.

Another substrate, namely a PAN fiber, eight harness satin graphitefabric substrate comprises a phenolic resin prepreg having about a 30%resin content and a fiber volume of between about 30% and about 35%. Thegraphite fabric of the substrate is heat-treated at between about 2000°C. and about 2300° C. prior to prepreg manufacture.

The preform laminate is formed by platen press or autoclave compactionand is appropriately cured and carbonized. Carbonization is to a maximumtemperature of 750° C. resulting in a disc substrate blank ready forpitch impregnation or CVI carbon processing. Disc blank density aftercarbonization is between 0.5 g/cc and 0.7 g/cc.

Carbon/Carbon Densification

In one form of the densification method of the invention the needled PANfiber carbon mat substrate is densified by chemical vapor infiltration(CVI) of carbon at a deposition temperature of approximately 1000° C. ata pressure of about 25 mm with a methane (CH₄) flow rate of about 170SCFH. The deposition time per run is between about 75 and about 120hours. Typically one or two infiltration runs will be required toachieve a carbon/carbon sample density after densification on the orderof about 1.15 g/cc to about 1.3 g/cc. Following CVI carbondensification, the carbon/carbon composite is heat treated at atemperature of between about 1600° C. and about 2500° C. The resultingcomposite is then machined to remove at least 0.050 inch per side fromeach friction surface and at least 0.250 inch per side from the insideand outside diameters.

In another form of the densification method of the invention the needledPAN fiber carbon mat substrate is densified by chemical vaporinfiltration (CVI) of carbon at a deposition temperature ofapproximately 1000° C. at a pressure of about 25 mm with a methane (CH₄)flow rate of about 170 SCFH. The deposition time per run is about 120hours. In this form of the method of the invention three or fourinfiltration runs are accomplished to achieve a carbon/carbon sampledensity after densification of on the order of about 1.5 g/cc to about1.6 g/cc.

Following CVI carbon densification, the carbon/carbon composite is onceagain heat-treated at a temperature of between about 1600° C. and about2500° C. The resulting composite is then machined to remove at least0.050 inch per side from each friction surface and at least 0.250 inchper side from the inside and outside diameters.

In still another form of the densification method of the invention theneedled PAN fiber carbon mat substrate is densified by chemical vaporinfiltration (CVI) of carbon at a deposition temperature ofapproximately 1000° C. at a pressure of about 15 mm with a methane (CH₄)flow rate of about 220 SCFH. The deposition time per run is once againbetween about 75 and about 120 hours. In this form of the method of theinvention three or four infiltration runs are accomplished to achieve acarbon/carbon sample density after densification of about 1.5 g/cc toabout 1.6 g/cc. Following CVI carbon densification, the carbon/carboncomposite is once again heat-treated at a temperature of between about1600° C. and about 2500° C.

The resulting composite is then machined to remove at least 0.050 inchper side from each friction surface and at least 0.250 inch per sidefrom the inside and outside diameters.

In yet another form of the densification method of the invention theneedled PAN fiber carbon mat substrate is densified by chemical vaporinfiltration (CVI) of carbon at a deposition temperature ofapproximately 1000° C. at a pressure of about 15 mm with a methane (CH₄)flow rate of about 220 SCFH. The deposition time per run is betweenabout 75 and about 120 hours. In this latest form of the method of theinvention four to six infiltration runs are accomplished to achieve acarbon/carbon sample density after densification of on the order ofabout 1.68 g/cc to about 1.73 g/cc. Following CVI carbon densification,the carbon/carbon composite is once again heat-treated at a temperatureof between about 1600° C. and about 2500° C. The resulting composite isthen machined to remove at least 0.050 inch per side from each frictionsurface and at least 0.250 inch per side from the inside and outsidediameters.

In one form of the densification method of the invention, the PAN fiber,eight harness satin graphite fabric substrate, following preformcarbonization, the laminate preform blank is pitch impregnated,carbonized again and then heat-treated to a temperature of approximately1700° C.

The low pressure pitch impregnation, carbonization and composite heattreatment (Lopic) cycles are repeated until the disc blank density is onthe order of about 1.15 g/cc to about 1.3 g/cc.

Following densification, the carbon/carbon composite is once againheat-treated at a temperature of between about 1600° C. and about 2500°C. The resulting composite is then machined to remove at least 0.050inch per side from each friction surface and at least 0.250 inch perside from the inside and outside diameters.

Final CVI Carbon, Post Ceramic Processing Run

The final chemical vapor infiltration run to deposit pyrolytic carboninto the carbon/ceramic composite blank is only employed after allceramic processing has been completed and the carbon/ceramic compositehas been exposed to its maximum process temperature.

-   -   CVI Carbon deposition temperature ˜1000° C.    -   Pressure—10 mm    -   Methane (CH₄) flow rate 220 SCFH    -   Deposition time per run—50 to 120 hours    -   Number of infiltration runs—1

Ceramic Phase Processing

By way of summary, two methods for ceramic hard phase insertion into thecarbon/carbon preform can be employed in the performance of the methodsof the invention. These are: pre-ceramic polymer impregnation andconversion to Silicon Carbide or by the CVI Carbide gas phase depositionprocess.

Ceramic Phase Processing by Pre-Ceramic Polymer Impregnation andPyrolyzation

The pre-ceramic polymer used for this process was developed by StarfireSystems, Inc. of Watervliet, N.Y. and is described in U.S. Pat. No.5,153,295 entitled “Carbosilane Polymer Precursors to Silicon CarbideCeramics.”

Detailed pre-ceramic polymer impregnation, curing, and pyrolyzationprocess information is available from Starfire Systems, Inc. and is setforth in an information sheet entitled “Curing and Pyrolyzing AHPCS SiCPrecursor Polymer Product Information Sheet.”

By way of example, pyrolysis done at 1000° C. will yield 80-85% ceramicmass. When pyrolysis is carried out at 1600° C., the mass yield willchange to 75-80%. Pyrolysis of AHPCS and related Hydridopolycarbosilanesto temperatures of 850° C. to 1,100° C. will result in a ceramic that isamorphous (non-crystalline) silicon carbide (SiC).

By pyrolyzing the part, in an inert atmosphere to 1600° C., with a holdtime at maximum temperature of 6-8 hours, the matrix is converted to acrystalline beta—SIC. The crystallization heating rate from roomtemperature to 1600° C. is 2° C./minute.

The following paragraphs set forth exemplary ceramic processes bypre-ceramic polymer impregnation and high temperature conversion toSilicon Carbide using Starfire Systems Inc. AHPCS SiC precursor polymer.The ceramic processes listed for Type A, Type B, and Type Ccarbon/ceramic friction materials, when matched with the appropriatedensity and porosity level of heat-treated carbon/carbon preforms (i.e.approximately 1.68 to 1.73 g/cc, 1.5 to 1.6 g/cc and 1.15 to 1.3 g/ccrespectively), are designed to result in three different frictionmaterials with either light level (Type A), medium level (Type B), orheavy level (Type C) ceramic hard phase in a heat-treated carbon/carbonbase material.

In all cases, in the final impregnation/pyrolysis (impreg/pyro) cyclefor each group of pre-ceramic densification lots, the discs will beexposed to at least 1600° C. in the pyrolyzation run.

Ceramic Process Sequence-Type A Carbon/Ceramic Material (2-3Impregnation/Pyrolysis (Impreg/Pyro) Cycles)

In carrying out Impreg/Pyro Cycle No. 1 of the Ceramic ProcessSequence-Type A, a selected carbon/carbon disc preform (such as a porousprecursor substrate formed by infiltrating a first carbon fibersubstrate with pyrolytic carbon, with a carbon/carbon preform densitylevel in the approximate range of 1.68 g/cc to 1.73 g/cc andheat-treated to between about 1600° C. and about 2500° C. to form aheat-treated carbon/carbon preform) is first vacuum impregnated with apre-ceramic polymer, such as the pre-ceramic polymer developed byStarfire Systems, Inc.

In accomplishing this step, before the polymer is introduced into thevacuum chamber, the chamber is evacuated to a vacuum of below about 250millitorr and is maintained at this vacuum level for about one hour perinch of thickness of the part. During the actual part impregnation step,the polymer is slowly introduced into the selected preform in a manneras to totally immerse the selected preform.

Once the part is thus impregnated to produce a first impregnatedpreform, the vacuum chamber is appropriately vented to atmosphere.

The next step in carrying out Impreg/Pyro Cycle No. 1 of Sequence-Type Ais to controllably cure the first impregnated preform.

This curing step is accomplished under about 150 pounds per square inch(psi) of inert gas, such as nitrogen with a cure heating rate ofapproximately 1° C. to 3° C./minute, depending on part thickness to atemperature of about 400° C. for approximately one hour.

Following the cure step, the thusly produced first cured preform ispyrolyzed. In accomplishing this step, an inert gas, such as nitrogen orargon is first caused to flow into the pyrolysis retort at a rate whichwill cause roughly one retort volume change every thirty minutes.

Once the retort is sufficiently purged, the cured preform is heated at arate of about 1° C. to 2° C./minute to approximately 1,000° C. and ismaintained at this temperature for approximately one hour to produce afirst impregnated and pyrolyzed preform (first impreg).

In carrying out Impreg/Pyro Cycle No. 2, the first impreg is vacuumimpregnated with the pre-ceramic polymer to form a second impregnatedpreform and cured in the manner described in the preceding paragraph toproduce a cured second impregnated preform. This done, cured secondimpregnated preform is pyrolyzed in the manner previously described atapproximately 1000° C. for about one hour to produce a pyrolyzed secondimpregnated preform. The pyrolyzed second impregnated preform is thenmachined to within 5% of final dimensions to prepare the secondimpregnated preform for the third impregnation cycle.

In carrying out Impreg/Pyro Cycle No. 3, the machined and pyrolyzedsecond impregnated preform is vacuum impregnated to form a furtherimpregnated third impregnated preform and cured in the manner describedin the preceding paragraphs to produce a cured third impregnatedpreform. This done, cured third impregnated preform is pyrolyzed in themanner previously described, but at a pyrolyzation temperature ofapproximately 1600° C. for about eight hours to form a third impregnatedand pyrolyzed preform. The third impregnated preform is then machinedflat and parallel removing approximately 0.025″ from all surfaces toform a fourth impregnated and pyrolyzed preform which is ready for postceramic processing.

Next, the fourth impregnated and pyrolyzed preform is heated to atemperature of about 1,000° C. to form a heated fourth impregnatedpreform and controllably infiltrated with methane (CH₄) in the mannerpreviously described at a pressure of about 10 mm and a methane flowrate of approximately 220 SCFH for about 50 to 120 hours to form a firstfriction disc.

Following the final CVI carbon run the first friction disc thus formedis machined to final dimensions to form a final friction disc. Thisdone, the final friction disc is tested in the following manner todetermine strength and friction characteristics.

Initially, the final friction disc is placed on a dynamometer in theconfiguration of rotors and stators in a multi-disc wheel set and testedunder typical aircraft braking conditions over a range of operatingconditions representative of landing, taxi, over-load and rejectedtakeoff conditions.

Additionally, the compressive strength, fracture toughness and thermalconductivity of the disc are tested. Finally, using microscopytechniques the level of the pyrolytic carbon deposition, carbonfiber/carbon matrix interface and the distribution of SiC within thedisc are determined and the density and the open porosity exhibitedthereby are determined, all in a manner well understood by those skilledin the art.

Ceramic Process Sequence-Type B Carbon/Ceramic Material (5-7 Impreg/PyroCycles)

In carrying out Impreg/Pyro Cycle No. 1 of the Ceramic ProcessSequence-Type B, a selected carbon/carbon disc or second carbon fiberdisc having a carbon/carbon preform density level of approximately 1.5g/cc to 1.6 g/cc, heat-treated to between about 1600° C. and about 2500°C., is first vacuum impregnated with a pre-ceramic polymer, such as thepre-ceramic polymer developed by Starfire Systems, Inc., in the samemanner as described in the Type A Carbon/Ceramic Material sequence toform an alternate first impregnated part. The next step in carrying outthe Type B Impreg/Pyro Cycle No. 1 is to controllably cure the alternatefirst impregnated part. This curing step is accomplished under about 150pounds per square inch (psi) of inert gas, such as nitrogen with a cureheating rate of approximately 1° C. to 3° C./minute, depending on partthickness to a temperature of about 400° C. for approximately one hour.

Following the cure step, the thusly produced cured alternate firstimpregnated part is pyrolyzed. In accomplishing this step, an inert gas,such as nitrogen or argon is first caused to flow into the pyrolysisretort at a rate which will cause roughly one retort volume change everythirty minutes.

Once the retort is sufficiently purged, the alternate cured part isheated at a rate of about 1° C. to 2° C./minute to approximately 1,000°C., and is maintained at this temperature for approximately one hour toproduce an alternate first impregnated and pyrolyzed preform.

In carrying out Impreg/Pyro Cycle No. 2, of the Type B Process Sequencethe alternate first impregnated preform is vacuum impregnated with thepre-ceramic polymer and cured in the manner described in the Type Asequence to produce a cured alternate second impregnated preform. Thisdone, the preform is pyrolyzed in the manner previously described atapproximately 100° C. for about one hour to produce a pyrolyzedalternate second impregnated preform. The pyrolyzed preform thus formedis then machined to within 5% of final dimensions to produce analternate third impregnated preform.

In carrying out the Impreg/Pyro Cycle No. 3, of the Type B ProcessSequence, the alternate second impregnated and machined preform isvacuum impregnated and cured in the manner described in the precedingparagraphs to produce a cured alternate third impregnated preform.

This done, cured preform is pyrolyzed in the manner previously describedat approximately 1000° C. for about one hour to produce an alternatethird impregnated and pyrolyzed preform.

In carrying out Impreg/Pyro Cycle No. 4, of the Type B Process Sequence,the alternate third impregnated preform is vacuum impregnated and curedin the manner described in the preceding paragraph to produce a curedalternate fourth impregnated preform. This done, cured alternate fourthimpregnated preform is pyrolyzed in the manner previously described atapproximately 1000° C. for about one hour to produce an alternate fourthimpregnated and pyrolyzed preform. This done, the alternate fourthimpregnated preform is machined to remove approximately 0.025″ from allsurfaces to prepare the alternate fourth impregnated and pyrolyzedpreform for the fifth impregnation cycle.

In carrying out Impreg/Pyro Cycle No. 5, of the Type B Process Sequence,the alternate fourth impregnated preform is vacuum impregnated and curedin the manner described in the preceding paragraphs to produce a curedalternate eighth impregnated preform. This done, the cured alternatefifth impregnated preform is pyrolyzed to about 1600° C. for about eighthours to produce an alternate fifth impregnated and pyrolyzed preform.This done, the alternate fifth impregnated preform is machined flat andparallel removing approximately 0.025″ from all surfaces to prepare thealternate fifth impregnated preform for the sixth impregnation cycle.

In one form of the method of the invention the alternate fifthimpregnated and pyrolyzed preform is formed into a second alternatefinal friction disc and tested in the manner previously described. Thetest data thusly developed is then compared with the earlier developedtest data.

In an alternate form of the method of the invention, or Impreg/PyroCycle No. 6, of the Type B Process Sequence, the alternate fifthimpregnated preform is vacuum impregnated and cured in the mannerdescribed in the preceding paragraph to produce a cured alternate sixthimpregnated preform. This done, cured alternate sixth impregnatedpreform is pyrolyzed to about 1000° C. for about one hour to produce analternate sixth impregnated and pyrolyzed preform.

In carrying out Impreg/Pyro Cycle No. 7, of the Type B Process Sequence,the alternate sixth impregnated preform is vacuum impregnated and curedin the manner described in the preceding paragraphs to produce a curedalternate seventh impregnated preform. This done, the cured alternateseventh impregnated preform is pyrolyzed to about 1600° C. for abouteight hours to produce an alternate seventh impregnated and pyrolyzedpreform.

Following Impreg/Pyro Cycle No. 7, of the Type B Process Sequence, thealternate seventh impregnated preform is machined to remove at least0.025″ from all surfaces to produce an alternate third friction discwhich is ready for post ceramic processing.

Following the final CVI carbon run, which is accomplished in the samemanner as discussed in the Ceramic Process Sequence-Type A. thealternate third friction disc is machined to final dimensions and testedin the same manner as discussed in the Ceramic Process Sequence-Type A.This done, the test results are carefully compared with the previouslydeveloped test results.

Ceramic Process Sequence-Type C Carbon/Ceramic Material (7-9 Impreg/PyroCycles)

In carrying out Impreg/Pyro Cycle No. 1 of the Ceramic ProcessSequence-Type C, a selected carbon/carbon disc preform or thirdprecursor preform with a carbon/carbon preform density level ofapproximately 1.15 g/cc to 1.3 g/cc heat-treated to between about 1600°C. and about 2500° C., is first vacuum impregnated with a pre-ceramicpolymer, such as the pre-ceramic polymer developed by Starfire Systems,Inc. in the same manner as described in the Type A Carbon/CeramicMaterial sequence.

The next step in carrying out the Impreg/Pyro Cycle No. 1, of the Type CProcess Sequence is to controllably cure the thusly impregnated part.

This curing step is accomplished under about 150 pounds per square inch(psi) of inert gas, such as nitrogen with a cure heating rate ofapproximately 1° C. to 3° C./minute, depending on part thickness to atemperature of about 400° C. for approximately one hour.

Following the cure step, the thusly produced first alternate-1 (alt.-1)cured part is pyrolyzed. In accomplishing this step, an inert gas, suchas nitrogen or argon is first caused to flow into the pyrolysis retortat a rate which will cause roughly one retort volume change every thirtyminutes.

Once the retort is sufficiently purged, the alt.-1 cured part is heatedat a rate of about 1° C. to 2° C./minute to approximately 1,000° C. andis maintained at this temperature for approximately one hour to producea first impregnated and pyrolyzed, alternate-1 preform (first alt-1impreg).

In carrying out Impreg/Pyro Cycle No. 2, of the Type C Process Sequence,the first alt.-1 impreg is vacuum impregnated with the pre-ceramicpolymer and cured in the manner described in the Type A sequence toproduce a cured second alt.-1 impreg. This done, cured second alt.-1impreg is pyrolyzed in the manner previously described at approximately1000° C. for about one hour to produce a pyrolyzed second alt.-1 impreg.The pyrolyzed second alt.-1 impreg is then machined to within 5% offinal dimensions to prepare the second alt.-1 impreg for the thirdimpregnation cycle.

In carrying out the Impreg/Pyro Cycle No. 3, of the Type C ProcessSequence, the second alt.-1 impreg is vacuum impregnated and cured inthe manner described in the preceding paragraphs to produce a curedthird alt.-1 impreg. This done, cured third alt-1.impreg is pyrolyzed inthe manner previously described at approximately 1000° C. for about onehour to produce a pyrolyzed third alt.-1 impreg.

In carrying out Impreg/Pyro Cycle No. 4, of the Type C Process Sequence,the third alt.-1 impreg is vacuum impregnated and cured in the mannerdescribed in the preceding paragraph to produce a cured fourth alt.-1impreg. This done, cured fourth alt.-1 impreg is pyrolyzed in the mannerpreviously described at approximately 1000° C. for about one hour toproduce a pyrolyzed fourth alt.-1 impreg.

This done, the fourth alt.-1 impreg is machined to remove approximately0.025″ from all surfaces to prepare the fourth alt.-1 impreg for thefifth impregnation cycle.

In carrying out Impreg/Pyro Cycle No. 5, of the Type C Process Sequence,the fourth alt.-1 impreg is vacuum impregnated to produce a fifth alt.-1impreg and cured in the manner described in the preceding paragraphs toproduce a cured fifth alt.-1 impreg. This done, cured fifth alt.-1impreg is pyrolyzed to about 1600° C. for about eight hours to produce apyrolyzed fifth alt.-1 impreg. This done, the fifth alt.-1 impreg ismachined flat and parallel removing approximately 0.025″ from allsurfaces to prepare the fifth alt.-1 impreg for the sixth impregnationcycle.

In carrying out Impreg/Pyro Cycle No. 6, of the Type C Process Sequence,the fifth alt.-1 impreg is vacuum impregnated and cured in the mannerdescribed in the preceding paragraph to produce a cured sixth alt.-1impreg. This done, cured sixth alt.-1 impreg is pyrolyzed to about 1000°C. for about one hour to produce a seventh alt.-1 impreg.

In carrying out Impreg/Pyro Cycle No. 7, of the Type C Process Sequence,the sixth alt.-1 impreg is vacuum impregnated and cured in the mannerdescribed in the preceding paragraphs to produce a cured seventh alt.-1impreg. This done, cured seventh alt.-1 impreg is heated to about 1000°C. for about one hour to produce a sixth pyrolyzed alt.-1 impreg.

In carrying out Impreg/Pyro Cycle No. 8, of the Type C Process Sequence,the seventh alt.-1 impreg is vacuum impregnated and cured in the mannerdescribed in the preceding paragraphs to produce a cured eighth alt.-1impreg. This done, cured eighth alt.-1 impreg is pyrolyzed in the mannerpreviously described at approximately 1000° C. for about one hour toproduce an eighth pyrolyzed alt.-1 impreg. This done the eighth alt.-1impreg is machined to remove approximately 0.025″ from all surfaces toprepare the eighth alt.-1 impreg for the ninth impregnation cycle.

In carrying out Impreg/Pyro Cycle No. 9, of the Type C Process Sequence,the eighth alt.-1 impreg is vacuum impregnated to produce a ninth alt.-1impreg and cured in the manner described in the preceding paragraphs toproduce a cured ninth alt.-1 impreg. This done, cured ninth alt.-1impreg is pyrolyzed to about 1600° C. for about eight hours to produce aninth pyrolyzed alt.-1 impreg. By pyrolyzing the part in an inertatmosphere to 1600° C. with a hold time of about eight hours, the matrixis converted to a crystalline beta-SiC.

This done, the ninth alt.-1 impreg is machined flat and parallelremoving approximately 0.025″ from all surfaces to produce a ninthmachined alt.-1 impreg.

Following Impreg/pyro cycle No. 9, the ninth alt.-1 impreg is now readyfor post ceramic processing.

Following the final CVI carbon run, which is accomplished in the samemanner as discussed in the Ceramic Process Sequence-Type A, the group ofdiscs is machined to final dimensions to produce a third group offriction discs. The third group of friction discs is then tested in thesame manner as discussed in the Ceramic Process Sequence-Type A, and thetest results are carefully compared with the test results of the firstand second groups of friction discs.

SEM images shown in FIG. 1 and FIG. 2 of the drawings illustrate atypical carbon/ceramic friction material manufactured according to thisinvention, using a light level of pre-ceramic polymer as SiC frictionmodifier source. For this material the needled PAN fiber carbon/carbonpreform had been CVI carbon densified to a density of 1.4 g/cc-1.6 g/ccand heat treated to 1600° C.-2500° C. The carbon/carbon preform had nextbeen exposed to three pre-ceramic polymer runs, cured and pyrolyzed.

Following the final pyrolyzation at 1600° C. and machining, thecarbon/ceramic composite had reached a density of 1.71 g/cc.

The photomicrographs show a light, uniform distribution of the beta-SiCthroughout the open pore structure of the composite.

Ceramic Phase Processing by Chemical Vapor Infiltration Technique

In yet another form of the invention a carbon/carbon preform withoptimized density and porosity level, heat treated to the appropriatetemperature, has the hard, ceramic friction modifier phase introduced bythe Chemical Vapor Infiltration (CVI) Process.

Ceramic Process Sequence-Type D Carbon/Ceramic Material (1-2 SiCChemical Vapor Infiltration Runs)

For this material concept a needled PAN fiber carbon mat substrate isdensified by chemical vapor infiltration (CVI) of carbon at a depositiontemperature of approximately 100° C. at a pressure of about 15 mm with amethane (CH₄) flow rate of about 220 SCFH. The deposition time per runis between about 75 and about 120 hours. In this form of the method ofthe invention three or four infiltration runs are accomplished toachieve a carbon/carbon sample density after densification of about 1.4g/cc to about 1.6 g/cc. Following CVI carbon densification, thecarbon/carbon composite is heat-treated at a temperature of betweenabout 1600° C. and about 2500° C.

The resulting composite is then machined to remove at least 0.050 inchper side from each friction surface and at least 0.250 inch per sidefrom the inside and outside diameters. The carbon/carbon preforms arenow ready for CVI SiC processing.

In carrying out SiC Chemical Vapor Infiltration Run No. 1 of the CeramicProcess Sequence-Type D, a selected heat-treated carbon/carbon discpreform, (such as a porous precursor substrate formed by infiltrating afirst carbon fiber disc with pyrolytic carbon, with a carbon/carbonpreform density level in the approximate range of 1.4 g/cc to 1.6 g/cc),is first stacked in the hot zone of a high temperature vacuum reactorwith spacers between each disc to allow access by the carbide forminggas phase to infiltrate the disc cross-section of each carbon/carbonpreform, reaching all open porosity throughout the discs.

The reactor hot zone is brought to the appropriate depositiontemperature with the required reactor pressure and gas flow rate toallow maximum part density to be reached for the selected infiltrationconditions and number of deposition hours selected.

Following cooling of the parts in an inert atmosphere, the first groupof infiltrated discs is removed from the reactor and machined to removeat least 0.025″ from all surfaces in preparation for the second CVI SiCreactor run.

In carrying out SiC Chemical Vapor Infiltration Run No. 2 of the CeramicProcess Sequence-Type D, the first SiC infiltrated preforms are againplaced into the SiC chemical vapor deposition reactor and infiltrated asdescribed for the SiC Chemical Vapor Infiltration Run No. 1 except thetime under deposition conditions is adjusted in order to achieve theas-infiltrated part density goal of approximately 1.6 g/cc-1.7 g/cc.

This done, the second group of infiltrated friction discs thus formed ismachined to final dimensions. Next, this group of friction discs istested to determine strength and friction characteristics.

The friction discs are placed on a dynamometer in the configuration ofrotors and stators in a multi-disc wheel set and tested under typicalaircraft braking conditions over a range of operating conditionsrepresentative of landing, taxi, over-load and rejected takeoffconditions. Additionally, the compressive strength, fracture toughnessand thermal conductivity of the discs are tested. Finally, usingmicroscopy techniques the level of the pyrolytic carbon deposition,carbon fiber/carbon matrix interface and the distribution of SiC withinthe discs are determined and the density and the open porosity exhibitedby the parts are determined, all in a manner well understood by thoseskilled in the art.

SEM images shown in FIG. 3, FIG. 4 and FIG. 5 of the drawings illustratea typical carbon/ceramic friction material manufactured according tothis invention, using CVI SiC as friction modifier source. For thismaterial the needled PAN fiber carbon/carbon preform had been CVI carbondensified to a density of approximately 1.4 g/cc and heat treated to1600° C.-2500° C. The carbon/carbon preform had next been exposed toonly one CVI SiC run. Following machining, the carbon/ceramic compositehad reached a density of 1.6 g/cc to 1.7 g/cc. The photomicrographs showa uniform distribution of SiC from the vapor phase throughout the openpore structure of the composite.

Using the aforementioned comparison of test results it is possible tocreate a new family of high performance friction materials with tailoredlevels of a ceramic hard phase to allow optimization of thermalconductivity, friction coefficient and wear performance ofcarbon/ceramic composite brake materials for use in the production ofnovel vehicle brakes.

By way of summary, reference to the drawings will further illustrate theconstruction of carbon/ceramic brake materials as manufactured inaccordance with the methods of the invention. The needled, PAN fiber,carbon/carbon preforms used in the materials illustrated, were firstdensified with pyrolytic, CVI carbon to a pre-determined density of 1.4g/cc-1.6 g/cc and heat-treated to 1600° C.-2500° C. prior to any ceramicprocessing.

The densification level for the carbon/carbon preform was specificallydesigned to assure that the required level of hard ceramic phase versussofter carbon/carbon phase was achieved in the final product.

Additionally, heat-treatment of the carbon/carbon preform was requiredto make sure that the thermal conductivity for the final carbon/ceramicfriction material was at the required level to achieve the friction andwear properties desired for a specific aircraft application.

The micro-structural images shown in the drawings, are for two ceramicmodified carbon/carbon preforms; one using a pre-ceramic polymer as SiCfriction modifier, pyrolyzed to 1600° C., (See FIG. 1 and FIG. 2) and asecond set of images illustrating carbon/carbon material with thefriction modifier introduced via the CVI SiC process (See FIG. 3, FIG. 4and FIG. 5).

FIG. 1 illustrates a needled PAN fiber substrate, carbon/carbon preformdensified with CVI carbon to a density of approximately 1.4 g/cc andheat-treated. The carbon/carbon preform was next exposed to threepre-ceramic polymer impregnation runs, cured and pyrolyzed. The finalpyrolyzation run was carried out at 1600° C. Following the threepre-ceramic polymer impregnation runs and pyrolyzation, the material hadreached an approximate density of 1.7 g/cc. The FIG. 1 image, at 100×,was generated by scanning electron microscope (SEM) back-scattertechnique. It illustrates uniform distribution of the beta-SiC phasethroughout the open pore structure of the needled, PAN fibercarbon/carbon preform.

FIG. 2 shows the same material as FIG. 1, but at a higher magnification.FIG. 2 was taken by SEM technique at 200× of a purposely-fracturedsample to allow observation of the carbon fiber/CVI carbon matrixinterface and the placement of the pyrolyzed SiC friction modifier inavailable open pores.

The particular sample shown had not yet been exposed to a final CVIcarbon densification run aimed at holding the ceramic friction modifierin place, while further improving the thermal conductivity of the endproduct.

The images shown in FIGS. 3, 4 and 5 also represent a carbon/carbonpreform based on a needled PAN fiber substrate. Again, the carbon/carbonpreform was densified to a specific density level and heat-treated. Thecarbon/carbon material shown in FIGS. 3, 4 and 5 had been CVI carbondensified to approximately 1.4 g/cc prior to ceramic processing. Thefriction modifier in this case is SiC introduced into the open pores ofthe carbon/carbon preform by chemical vapor infiltration technique.

FIG. 3 shows an even penetration and distribution of the CVI SiCfriction modifier product throughout the available open pores. The SiCis deposited on the pore walls and available fiber surfaces. The CVI SiCshown in FIG. 3, FIG. 4 and FIG. 5 had been deposited by one chemicalvapor infiltration run, resulting in a material density of approximately1.6 g/cc to 1.7 g/cc.

FIG. 4 and FIG. 5 are images at higher magnification (2000× and 5000×),again of the same material, but of a purposely-fractured surface.

FIG. 4 and FIG. 5 illustrate the CVI carbon matrix deposited around thecarbon fibers, it also illustrates the uniform over-coating of porewalls and fibers in the carbon/carbon preform with a thin layer of CVISiC friction modifier.

Having now described the invention in detail in accordance with therequirements of the patent statutes, those skilled in this art will haveno difficulty in making changes and process modifications in theindividual parts or their relative assembly in order to meet specificrequirements or conditions. Such changes and modifications may be madewithout departing from the scope and spirit of the invention as setforth in the following claims.

1. A method of making a carbon/ceramic matrix composite having aspecific carbon/ceramic composition with tailored levels of a ceramichard phase and a soft carbon/carbon phase to achieve optimum thermalconductivity, friction coefficient and wear performance and correlatingthe resulting composite properties with vehicle braking performancerequirements, the method comprising the steps of: (a) infiltrating afirst carbon fiber substrate with pyrolytic carbon to reach the specificcarbon/carbon density of approximately 1.68 g/cc to approximately 1.73g/cc with appropriate porosity level suitable for impregnation with apre-ceramic polymer to form a carbon/carbon preform and heating saidcarbon/carbon preform to between about 1600 degrees C. and about 2500degrees C. to form a heat-treated carbon/carbon preform; (b)impregnating said heat-treated carbon/carbon preform with a pre-ceramicpolymer to form a first impregnated preform; (c) heating said firstimpregnated preform within an inert gaseous atmosphere to a temperatureof about 400 degrees C. for about one hour to form a first curedpreform; (d) heating said first cured preform to a temperature of about1,000 degrees C. for about one hour to form a first impregnated andpyrolyzed preform; (e) impregnating said first impregnated and pyrolyzedpreform with a pre-ceramic polymer to form a second impregnated preform;(f) heating said second impregnated preform within an inert gaseousatmosphere to a temperature of about 400 degrees C. for about one hourto form a cured second impregnated preform; (g) heating said curedsecond impregnated preform to a temperature of about 1,000 degrees C.for about one hour to form a pyrolyzed second impregnated preform; (h)machining said pyrolyzed second impregnated preform to prepare saidpyrolyzed second impregnated preform for a third impregnation cycle toform a machined pyrolyzed second impregnated preform; (i) impregnatingsaid machined pyrolyzed second impregnated preform with a pre-ceramicpolymer to form an impregnated third impregnated preform; (j) heatingsaid third impregnated preform within an inert gaseous atmosphere to atemperature of about 400 degrees C. for about one hour to form a curedthird impregnated preform; (k) heating said cured third impregnatedpreform to about 1600 degrees C. for about eight hours to produce apyrolyzed third impregnated preform; (l) machining said pyrolyzed thirdimpregnated preform to form a fourth impregnated preform; (m) heatingsaid fourth impregnated preform to a temperature of about 1000 degreesC. to form a heated fourth impregnated preform; (n) infiltrating saidheated fourth impregnated preform with methane gas to form a firstfriction disc; (o) machining said first friction disc to finaldimensions to form a final friction disc; (p) subjecting said finalfriction disc to dynamic testing to determine its strength and frictioncharacteristics; and (q) correlating said strength and frictioncharacteristics with vehicle braking performance requirements.
 2. Themethod as defined in claim 1 in which the first carbon fiber substrateis constructed from rayon precursor fibers.
 3. The method as defined inclaim 1 in which the first carbon fiber substrate is constructed frompolyacrillonitrile fibers.
 4. The method as defined in claim 1 in whichthe first carbon fiber substrate is constructed from pitch fibers. 5.The method as defined in claim 1 in which the first carbon fibersubstrate is constructed from three dimensional, needled carbon fiberpreforms.
 6. The method as defined in claim 1 in which the first carbonfiber substrate comprises an eight harness satin graphite fabriclaminate.
 7. The method as defined in claim 1 in which the first carbonfiber substrate comprises a chopped graphite fiber molding compound. 8.The method as defined in claim 1 in which said final friction disc isfurther tested to determine its compressive strength, fracture toughnessand thermal conductivity.
 9. The method as defined in claim 1 in whichsaid final friction disc is further tested to determine the density anddegree of porosity of said final friction disc.
 10. The method asdefined in claim 1 further comprising the steps of; (a) infiltrating asecond carbon fiber substrate with pyrolytic carbon to reach thespecific carbon/carbon density of approximately 1.15 g/cc to 1.3 g/ccwith appropriate porosity level and heat-treating the carbon/carbonpreform to between about 1600° C. and about 2500° C. suitable forimpregnation with a pre-ceramic polymer to form a second porousprecursor substrate; (b) impregnating said second porous precursorsubstrate with a pre-ceramic polymer to form an alternate firstimpregnated part; (c) heating said alternate first impregnated partwithin an inert gaseous atmosphere to a temperature of about 400 degreesC. for about one hour to form a cured alternate first impregnated part;(d) heating said cured alternate first impregnated part to a temperatureof about 1,000 degrees C. for about one hour to form an alternate firstimpregnated and pyrolyzed preform; (e) impregnating said alternate firstimpregnated preform with a pre-ceramic polymer to form an alternatesecond impregnated preform; (f) heating said alternate secondimpregnated preform within an inert gaseous atmosphere to a temperatureof about 400 degrees C. for about one hour to form a cured alternatesecond impregnated preform; (g) heating said cured alternate secondimpregnated preform to a temperature of about 1,000 degrees C. for aboutone hour to form an a pyrolyzed alternate second impregnated preform;(h) machining said pyrolyzed alternate second impregnated preform toprepare the preform for the third impregnation cycle; (i) impregnatingsaid alternate second impregnated and machined preform with apre-ceramic polymer to form an alternate third impregnated preform; (j)heating said alternate third impregnated preform within an inert gaseousatmosphere to a temperature of about 400 degrees C. for about one hourto form a cured alternate third impregnated preform; (k) heating saidalternate third impregnated preform to about 1000° C. for one hour toproduce a pyrolyzed alternate third impregnated preform; (l)impregnating said pyrolyzed alternate third impregnated preform with apre-ceramic polymer to form an alternate fourth impregnated preform; (m)heating said alternate fourth impregnated preform within an inertgaseous atmosphere to a temperature of about 400° C. for about one hourto form a cured alternate fourth impregnated preform; (n) heating saidcured alternate fourth impregnated preform to a temperature of about1,000 degrees C. for about one hour to form an alternate fourthpyrolyzed impregnated preform; and (o) machining said alternate fourthpyrolyzed impregnated preform to prepare the preform for the fifthimpregnation cycle.
 11. The method as defined in claim 10 in which saidsecond porous precursor substrate is constructed from rayon precursorfibers.
 12. The method as defined in claim 10 in which said secondporous precursor substrate is constructed from polyacrillonitrilefibers.
 13. The method as defined in claim 10 in which the second carbonfiber substrate is constructed from pitch fibers.
 14. The method asdefined in claim 10 in which second porous precursor substrate isconstructed from three dimensional, needled carbon fiber_preforms. 15.The method as defined in claim 10 in which the second carbon fibersubstrate comprises a chopped graphite fiber molding compound.
 16. Themethod as defined in claim 10 in which the second carbon fiber substratecomprises eight harness satin graphite fabric.
 17. The method as definedin claim 10 further comprising the steps of: (a) impregnating saidalternate machined fourth impregnated preform with a pre-ceramic polymerto form an alternate fifth impregnated preform; (b) heating saidalternate fifth impregnated preform within an inert gaseous atmosphereto a temperature of about 400 degrees C. for about one hour to form analternate cured fifth impregnated preform; (c) heating said alternatecured fifth impregnated preform to about degrees C. for about eighthours to produce an alternate fifth pyrolyzed impregnated preform; (d)machining said alternate fifth impregnated preform to form an alternatefinal friction disc; (e) subjecting said second alternate final frictiondisc to dynamic testing to determine its strength and frictioncharacteristics; and (f) comparing the dynamic testing of said alternatefinal friction disc with the dynamic testing of said final frictiondisc.